1. Field of the Invention
The present invention relates to a projection optical system for projecting a magnified image on a screen, and to a rear projection optical system provided with such a projection optical system. More specifically, the present invention relates to an oblique projection optical system and a rear projection optical system that shine a beam of light on a screen from an oblique direction.
2. Description of the Prior Art
From long ago, it has been common practice to project a magnified image of an image displayed on a small display surface onto a screen. Before, projection of an image on a screen was generally achieved through front projection, whereby the image is projected from in front of the screen, i.e. from the same side as the observer, for example as when a movie is shown in a movie theater. These days, projection of an image is achieved also through rear projection, whereby the image is projected from behind the screen by the use of a screen that transmits light. Today, large-screen television sets adopting rear projection are in practical use.
It is desired that, except in cases where a large facility like a movie theater itself constitutes a projection apparatus, a projection apparatus be provided with a large screen and be simultaneously compact. In particular, in a rear projection apparatus that projects an image from behind a screen, it is desired that the apparatus be slim, i.e. that its depth dimension in the direction perpendicular to the screen be small.
In early models of rear projection apparatus, to make them slim, a very common centered optical system is used as a projection optical system, and a flat-surface mirror is arranged behind a screen so as to turn the optical path of the light exiting from the powered part of the projection optical system. However, to prevent distortion in the image formed on the screen, the optical path along the optical axis turned by the flat-surface mirror needs to run through the center of the screen perpendicularly thereto. This makes it difficult to slim down the apparatus below a certain thickness. The optical path is turned vertically, because then the turned optical path is shorter than if it is turned horizontally, and usually all the parts, including the display surface on which an image is displayed, other than the flat-surface mirror for turning the optical path are arranged below the screen.
An effective way to further slim down rear projection apparatus is to adopt oblique projection, in which the ray of light striking the center of the screen, i.e. the ray representing the center of the image, is incident on the screen at a large angle of incidence. However, attempting to achieve oblique projection with a centered projection optical system necessitates making the optical path along the optical axis turned by the flat-surface mirror run off the center of the screen. Accordingly, the projection optical system needs to include a large-diameter wide-angle lens of which only part is used for projection. Such an optical system can be realized, but it entails high cost, and in addition makes the projection optical system itself larger, with little effect of slimming down the apparatus.
To overcome this, proposals have been made to use reflecting mirrors with curved surfaces as powered elements included in the projection optical system. For example, Re-published Patent Application No. WO 97/01787 proposes a projection optical system composed of four curved-surface mirrors. These curved-surface mirrors have, in order from the display surface side, a positive, a negative, a positive, and a negative power. The curved surface closest to the display surface is a spherical surface, and the other three curved surfaces are aspherical surfaces. The projection optical system that the applicant of the present invention proposes in Japanese Patent Application Laid-Open No. 2001-221949 also is composed of four curved-surface mirrors. In this projection optical system, the curved-surface mirrors have, in order from the display surface side, a positive, a positive, a negative, and a negative power, or a positive, a positive, a negative, and a positive power. All these surfaces are spherical or aspherical surfaces. In addition to these publications, there are more that propose projection optical systems composed of three curved-surface mirrors and of other types.
Conventionally, an oblique projection optical system composed of curved-surface mirrors is, to minimize the lowering of imaging performance, designed to have a large f-number, and has a long optical path length from the display surface on which an image is displayed to the projection surface at which a screen is arranged. Moreover, to slim down the apparatus incorporating it while securing a long optical path length, its optical path is turned many times with flat-surface mirrors. The optical path needs to be turned, except on the last occasion, around the screen, specifically below or above the screen, so as not to hinder the projection of the image on the screen. Thus, an oblique projection optical system composed of curved-surface mirrors helps slim down the apparatus incorporating it, but does not contribute to reducing the height dimension thereof. Moreover, in a conventional oblique projection optical system, only necessary parts of curved-surface mirrors are used so as not to hinder miniaturization. Anyway, all these mirrors have spherical or aspherical surfaces that are symmetric about an axis.
As long as a long optical path length is secured to prevent the lowering of imaging performance, it is difficult to reduce the height dimension of the screen without sacrificing the flatness of the apparatus. Thus, modern oblique projection optical systems are considered to have almost reached the limit in terms of the trade-off between the slimming-down of projection apparatus and the reduction of the height dimension thereof.
On the other hand, as described above, rear projection optical systems used in common rear projection apparatus achieve the slimming-down of the apparatus by turning the optical path of the light exiting from a projection optical system with a single reflecting mirror arranged behind a screen. However, the projection optical system used here is of a centered type, and therefore the ray striking the center of the screen surface needs to be substantially perpendicular to the screen surface. This makes it difficult to slim down rear projection optical systems below a certain thickness.
To overcome this, various optical arrangements have been proposed for further slimming-down. For example, Japanese Patent Registered No. 2932609 and Japanese Patent Applications Laid-Open No. H3-87731, H2-153338, H2-146535, and H2-130543 disclose rear projection optical systems in which the optical path of a projection optical system is turned with two flat-surface reflecting mirrors.
However, with conventional rear projection optical systems, sufficient slimming-down is difficult, or slimming them down poses new problems. For example, the rear projection optical system disclosed in Japanese Patent Registered No. 2932609 mentioned above adopts a method using a re-imaging projection optical system in which a displayed image is first imaged, and the resulting image is then projected on a screen surface so as to be imaged again. This inevitably makes the projection optical system large. In addition, this method requires an oblique projection optical system that permits the ray striking the center of the screen surface to be incident thereon at a large angle of incidence, but the publication describes no specific optical arrangement of such an optical system. The rear projection optical systems disclosed in Japanese Patent Applications Laid-Open No. H3-87731, H2-153338, H2-146535, and H2-130543 mentioned above also require oblique projection optical systems for slimming-down, but the publications do not make it clear what specific optical arrangement to use as a projection optical system.
An oblique projection optical system is usually realized by using part of a centered optical system. However, to slim down a rear projection optical system, the projection angle of the principal ray needs to be made very large. Thus, it is inevitable to use part of a very wide-angle centered optical system. In general, a wide-angle optical system requires a large number of lens elements, and their lens diameters are very large. This makes the optical system as a whole large.
Some display apparatus incorporate a rear projection optical system that is slimmed down by actually adopting an oblique projection optical system employing curved-surface reflecting mirrors. In these display apparatus, however, the light that has exited from the projection optical system is reflected directly by a flat-surface reflecting mirror arranged behind a screen, and thus the curved-surface reflecting mirror that constitutes the last surface of the projection optical system needs to be very large. A large curved-surface reflecting mirror like this is disadvantageous in terms of mass production and cost. Moreover, if the projection optical system includes only three curved-surface mirrors, it is highly sensitive to errors, and is thus difficult to manufacture.